1. Field of the Invention
This invention relates to the field of metallurgy, and particularly to the field of processing precipitation hardening aluminum alloys.
2. Description of the Prior Art
A fine grain size tends to improve the mechanical properties of most structural materials. Additionally, formability can be improved by elimination of "orange peel" structure, and superplasticity realized in many alloys by providing a fine grain structure. For alloys which are susceptible to stress corrosion cracking such as many precipitation hardening aluminum alloys, a fine grain structure generally decreases the susceptibility to stress corrosion. However, grain refinement is difficult to achieve in aluminum alloys, and most attempts to obtain a fine grain size by conventional mechanical working and recrystallization by heating have only resulted in the material recrystallizing to the original coarse grain size with large "pancake" shaped grains.
U.S. Pat. No. 4,092,181 to Paton and Hamilton describes a method of imparting a fine grain to precipitation hardening aluminum alloys. Patent Application Ser. No. 62,203 filed July 30, 1979 by Hamilton, Mahoney, and Paton, now U.S. Pat. No. 4,222,797 describes an improvement in the earlier method which utilizes an increased rate of forming to obtain even finer grains. According to both these prior methods, the aluminum alloy is mechanically worked below its recrystallization temperature and then heated to above the recrystallization temperature. These prior methods work well for sheet and thin plate which can be heated rapidly to above the recrystallization temperature. However, heavy plate and large bars heat up slowly because of their mass, and the worked material may recover before the recrystallization temperature is reached. Consequently, expensive, fast heat up procedure such as the use of molten salt and lead baths are required to obtain fine grain in heavy sections of aluminum.